Preparation Of La_0.71Ca_0.29Mn_1-xB_xO_{3< /sub>(B=Cr,Fe,Ti,Co,V) And Its Electromagnetic Properties}

Perovskite manganese oxides have colossal magneto resistance effect and metal-insulator phase transitions accompanying ferromagnetism,which make them have important applications in magnetic memory,infrared detector,resistance transducer and bolometer.However,these materials have a lower temperature coefficient of resistance?TCR?and a narrower range of metal-insulator transition temperatures?T_p?,which limit their practical application.Therefore,how to improve the TCR and T_p of materials has become the hotspot of research.Based on the previous results,La_0.71Ca_0.29MnO₃ was employed as the parent phase,the effect of low doping at Mn-site on the crystal structure,surface morphology and electrical transport properties of polycrystalline ceramics was studied.La_0.71Ca_0.29Mn_1-xB_xO₃?B=Cr,Fe,Co,Ti,V?polycrystalline ceramics were synthesized by sol-gel method with methanol as solvent.The physical properties of the samples were characterized by various analized methods.all samples illustrated orthorhombic structure with Pnma space group.The sharp diffraction peaks indicate that the polycrystalline samples have high crystalline quality.The slight amount of the elements?Cr,Fe,Co,Ti,V?substituting for Mn-site does not modify the perovskite structure of the sample,but can distort the crystal structure by affecting Mn-O-Mn bond angle and length.The proper amount of doping element is beneficial to crystal grains'growth,while larger amount will lead to crystal grains shrinking and the appearance of pores.With the increase of doping amount of elements?Cr,Fe,Co,Ti,V?at the Mn-site,the metal-insulator transition of the sample shifts to the lower temperature,meanwhile the resistivity increases.Cr and V can improve the TCR of the sample at low doping.In addition,samples doped with Fe and Ti exhibit insulator behavior at x=0.10.The magnetoresistance?MR?of Cr and V doped samples were tested.The magnetoresistive test results show that under the applied magnetic field,the doping of the element can increase T_p and MR of the sample and reduce its resistivity.The change of these properties is mainly attributed to the fact that the doping of the elements changes the ratio of Mn³⁺/Mn⁴⁺,and affects the double exchange interaction,Jahn-teller effect and carrier density in the system.Ag was added to La_0.71Ca_0.29Mn_0.97B_0.03O₃?B=Cr,Fe,Co,Ti,V?series samples.It can be seen from the test results that the addition of Ag can change the surface morphology of the samples and promote the growth of the crystal grains,but the roughness of the microscopic interface of the sample will increase.On the other hand,the addition of Ag not only improves T_p and TCR(up to 65.1%·K^-1)of some samples,but also significantly reduces the resistivity.This may be attributed to the fact that partial Ag is oxidized to replace La or Ca,and Ag exist at grain boundaries and surfaces,resulting in change in the electrical transport behavior.